The present invention relates to an image forming apparatus for forming an image on a recording sheet by transferring a toner image, which is formed on an image carrier member, such as a photoreceptor drum, onto the recording sheet. More particularly, the invention relates to an image forming apparatus of the type in which a recording sheet is transported to a toner image transferring stage in a state that the recording sheet is electrostatically supported to a transfer-member supporting member.
In a known image transferring system for transferring a toner image from an image carrier member, for example, a photoreceptor drum, onto a recording sheet, a transfer-member supporting member, such as a dielectric film, disposed between a latent electrostatic image carrier member and a transfer charging means. Corona charge is applied to the rear side of the transfer-member supporting member with a recording sheet that is put into close contact therewith. The recording sheet is electrostatically attracted to the transfer-member supporting member when the latter is charged. By utilizing this feature, the image transferring system may have the three functions; to transfer the toner image onto the recording sheet, to transport the recording sheet, and to separate the recording sheet from the latent electrostatic image carrier member.
This image transferring system is adequate to a color copying machine which requires the function to transfer a multiple of color toner images onto a recording sheet. In the conventional color copying machine, the image transfer drum with the transfer-member supporting member wound thereon is turned in synchronism with the image carrier member. Color toner images of black, yellow, cyan, and magenta are repeatedly transferred onto the recording sheet attractively supported by the transfer-member supporting member.
FIGS. 14A and 14B specifically show this toner image transferring system, which is incorporated into the image transfer stage. In the figure, reference numeral 100 designates an image carrier member; numeral 101, a transfer-member supporting member; 102, a recording sheet; and 103, a transfer charger. As shown in FIG. 14A, the recording sheet 102 arrives at the image transfer stage in a state that the recording sheet 102 is closely put on the transfer-member supporting member 101. At this time, the transfer charger 103 charges the transfer-member supporting member 101 from its reverse side, whereby the transfer-member supporting member 101 is charged with the charge of the polarity that is opposite to the polarity of toner 104. The transfer charger 103 starts the charging operation when the leading edge of the recording sheet reaches the image transferring position, in order to prevent a poor image transfer. In the instance illustrated, the toner for forming a toner image on the image transferring means is negatively charged, while the transfer-member supporting member 101 is positively charged by the transfer charger 103. Accordingly, the toner 104 is transferred from the image carrier member 100 to the recording sheet 102 by the electrostatic attraction, to thereby form a toner image on the recording sheet 102 (FIG. 14B).
Following the transfer of the toner image, the image carrier member moves to a cleaning stage. In this stage, toner left on the surface of the image carrier member is removed by a cleaner. It is a common practice that a pre-cleaning charge removal means for discharging the cleaner is provided at the prestage of the cleaner. A space around the image carrier member is limited. Therefore, the pre-cleaning charge removal means must be located close to the toner image transferring stage.
For this reason, the image forming apparatus based on the above-mentioned image transferring system inherently suffers from the following disadvantages. After the toner image is transferred, as shown in FIG. 15, an excessive amount of charge of the polarity (positive in the illustrated instance) that is opposite to that of the toner 104 is present on the rear side of the transfer-member supporting member 101 after it passes the transfer stage. Accordingly, the charge of the polarity (negative in the illustrated instance) that is the same as that of the toner, which is generated by the pre-cleaning charger 105, is excessively attracted to the recording sheet. The result is to weaken the force to attach the toner image to the recording sheet 102. When that force is weakened, toner of the toner image attached to the recording sheet tends to scatter during the transportation of the recording sheet. The scattering of toner impairs the quality of the transfered toner image. The scattered toner attaches to the component parts disposed around the transfer-member supporting member. In recent quality, color copying machine, the scattering of toner becomes more problematic since the machine uses finer toner of remarkably reduced toner particle diameters.
To solve the problems, another image forming apparatus is disclosed in Published Unexamined Japanese Patent Application Hei-2-247672. In the apparatus, the transfer-member supporting member 101 is surrounded with a conductive plate applied with a voltage of the same polarity as of the toner. Provision of the conductive plate prevents the toner image from being peeled off. The image forming apparatus is still defective, however. Since the transfer-member supporting member is entirely covered with the conductive plate, the size of the image forming apparatus is increased. The electrostatic force acts between the transfer-member supporting member and the recording sheet. The electrostatic force of the recording sheet acting on the transfer-member supporting member is decreased. A problem will arise in the transport of the recording sheet.
One of the effective ways to reduce the amount of charge attracted to the recording sheet by the pre-cleaning charger, is to decrease the voltage applied to the pre-cleaning charger. However, the decreased voltage impairs the ability of the pre-cleaning charger to remove the charge from the image carrier member. When the trailing edge of the recording sheet leaves a location between the transfer-member supporting member and the transfer-member supporting member, a large discharge takes place, so that the surface potential of the image carrier member goes positive in polarity. Therefore, the pre-cleaning charger of the impaired charge removing ability can insufficiently remove the charge from the transfer-member supporting member, particularly a surface region of the image carrier member having been put into contact with the trailing edge of the recording sheet in the image transfer stage.
An approach to reduce the force to pull the charge of the same polarity as of the toner from the pre-cleaning charger to the recording sheet, by removing an excessive amount of charge applied from the transfer charge to the transfer-member supporting member, is also effective for reducing the amount of the charge going to the recording sheet, from the pre-cleaning charger. The charge of the transfer-member supporting member naturally reduces with time after the image transfer. If charge is removed from the transfer-member supporting member immediately after it passes the image transfer stage, the electrostatic force of the recording sheet that acts on the transfer-member supporting member reduces. As a result, a problem will arise in the transportation of the recording sheet. Particularly, in the color copying machine in which the transfer drum is rotated while supporting the recording sheet, when the electrostatic force acting between the recording sheet and the transfer-member supporting member is reduced, the recording sheet slips out of place, so that the transferred color toner images are out of register.